runtime(vim): Update base-syntax, improve variable matching

- Match variables after operators, including line continuations.
- Match option variables without leading whitespace.
- Explicitly match expression subscripts.
- Match Vim9 variables in LHS of assignments and method calls.
- Match option variables (&option) with a dedicated syntax group like
  environment variables.
- Match list literals, fixes: #5830
- Match :{un}lockvar arguments.
- Match registers and environment variables in :let unpack lists.
- Match lambda expressions
- Match Vim9 scope blocks
- Match variables in :for subject
- Highlight user variables with Normal
- Improve this/super keyword matching, fixes: #15970

closes: #16476

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/dumps/vim_ex_substitute_08.dump

@@ -1,5 +1,5 @@
 |d+0#af5f00255#ffffff0|e|f| +0#0000000&|T|e|s|t|(+0#e000e06&|)| +0#0000000&@64
-@2|s|t|r|[|s|]| @66
+@1| +0#00e0e07&|s|t|r|[+0#0000000&|s+0#00e0e07&|]+0#0000000&| @66
 @2|s|t|r|(+0#e000e06&|s+0#00e0e07&|)+0#e000e06&| +0#0000000&@66
 |e+0#af5f00255&|n|d@1|e|f| +0#0000000&@68
 > @74